The invention relates to a wireless local loop system comprising base stations and an access network node which connects the cordless terminal equipments to the exchange of the fixed network. The system especially comprises an air interface between the cordless terminal equipment and the base station, the interface being mainly in accordance with the GSM mobile system, wherein a multiframe of a traffic channel TCH consists of 26 TDMA frames, at least one of which is a frame of a slow associated control channel (SACCH) in the time slot of which the terminal equipment (T) transmits measurement information to the base station and the base station (BS) transmits control information to the terminal equipment, and one frame is an IDLE frame.
When a telephone network is being built, the installation of the subscriber lines between the exchange and the subscriber equipments is not only considerably expensive but also requires a great deal of time. Usually the network of subscriber lines is formed in such a way that twin cables beginning in several subscriber equipments are supplied to a distribution frame, and cables originating in several crossbar switches are combined in another distribution frame the cable of which is supplied to the exchange. The signalling interface between the exchange and the subscriber lines is standardized, and it is either an interface of two-wire analogous subscriber lines, a multiplexer interface according to Recommendation V2 of the CCITT, or a message-based multiplexer interface according to Recommendation V5.1 of the ETSI. Altering fixed subscriber lines is troublesome, and maintenance costs especially in areas where the subscriber lines are overhead lines can be considerable. A solution to these problems is replacing the fixed lines between the exchange and the subscriber equipments with radio lines. The solution is known as a wireless local loop (WLL) system.
The principle of the WLL system is shown in FIG. 1. A wireless stationary terminal equipment T comprises a radio unit 4 provided with an antenna, and a telephone adapter that connects a standard subscriber equipment 5 to the terminal equipment. The subscriber equipment may be a normal telephone set, a telefax terminal, or a modem. It is attached to the terminal equipment by inserting a standard plug in the adapter connection of the terminal equipment. The user utilizes the subscriber equipment 5 in the same way as in a conventional fixed network, even though the subscriber line connection consists of a radio line between the terminal equipment T and the base station BS 2 or 3. The base station may serve several subscriber equipments. The base station is connected to a special access network node 1, which in turn is connected to a standard exchange. Several base stations may be connected to one access network node 1.
The WLL system may be built by using components of an existing mobile phone system. The mobile system may be for example an analogous NMT system or a digital GSM system. In such a case, the signalling of the WLL system is in accordance with the system concerned, the base stations are standard base stations of this system, and the radio unit of the terminal equipment is similar to the radio unit of the mobile station or the terminal equipment may be a mobile station in a mobile system. An important component in the WLL system is the access network node that connects the subscribers to the standard local exchange. The access network node converts the WLL network signalling, for example NMT or GSM signalling, into signalling suitable for the fixed network (e.g. PSTN), and correspondingly it adapts the signalling of the fixed network to the WLL network interface.
The network node is connected to the local exchange with an open V2- or V5.1-type multiplexer interface that utilizes a 2 Mbit/s PCM system. If the local exchange only supports the two-wire interface, the network node is connected to the exchange by converting the V2 signalling into an analogous two-wire subscriber loop interface by means of a multiplexer. The signalling between the network node and the base stations connected thereto is the signalling of an adapted mobile network, but it is modified in such a way that the functions, such as cell handover and roaming, typical of the cellular network are prevented. Therefore, the subscriber must remain within the coverage area of the base station assigned to it. The routing of an incoming call and an outgoing call is based on the subscriber database of the network node. The operation of the network node corresponds to the operation of a concentrator: a call is forwarded from a subscriber interface to the exchange, and the analysis of the numbers, calculation and other functions are performed in the exchange.
According to what is described above, a WLL network may be based on a known GSM system. The GSM is a digital cellular system based on time division multiple access. The channels of the system will be described below.
Logic channels are divided into traffic channels TCH that transmit speech and data, and into control channels that forward signalling and synchronization data. Traffic channels include a full rate traffic channel TCH/F and a half rate traffic channel TCH/H. The full rate traffic channel is always allocated together with a slow associated control channel SACCH. Therefore one carrier wave can be used to form eight TACH/F channels when all the time slots of the TDMA frame are allocated to traffic channels.
The TCH/F+SACCH/TF multiframe shown in FIG. 2 comprises 26 successive frames, in which the TCH/F cycle of 26 time slots comprises 24 time slots in which the TCH/F burst is transmitted, one time slot in which the SACCH burst is transmitted, and one time slot, IDLE, in which no transmission occurs. According to the figure, the SACCH burst is transmitted in the time slot of frame 12, and no transmission occurs in the time slot of frame 25 IDLE.
A mobile station utilizes the SACCH for forwarding different measurement results to the base station. The base station transmits commands related to power control and timing advance to the mobile station on the control channel. Further, the base station transmits general network information, so-called system.sub.-- infos 5 and 6. This information contains a list of frequencies of neighbouring cells to be monitored, the base station identity code BSIC, the BCCH frequency data, parameters relating to interference control of the radio line, etc. The information is also transmitted on the broadcast control channel (BCCH). The SACCH message contains a so-called Layer 1 Header wherein the timing advance and power control commands are transmitted in the downlink direction, and acknowledgment of the aforementioned commands is forwarded in the uplink direction.
The mobile station and the base station must measure the power of the signal they have received, and the mobile station must also measure the power of the BCCH carrier of the neighbouring base stations. The quality of the received signal is also measured by calculating the bit error ratio. The mobile station must report the results of the measurement to its own base station. It reports them on the aforementioned SACCH. According to the GSM recommendation, the measurement results must be reported at least once a second. This means that the cycle of measurement and reporting contains four multiframes, 480 ms in time, during which the measurements are performed, and another four multiframes, 480 ms in time, during which the measurement results are transmitted. If the SACCH is only utilized for reporting, the mobile station can report the measurement results twice a second. The measurement and reporting cycle contains four multiframes, wherefore the results are transmitted every 480 ms. The mobile station manages to perform the decoding of the BCCH signal of the neighbouring cells only during the time slot of the last, i.e. idle, frame in the multiframe (FIG. 2).
The base station uses the measurement results for controlling both the power of its own transmitter and the power of the mobile station, for providing the mobile station with the timing advance, and for handover purposes. The purpose of power control both in the uplink and downlink direction is, firstly, to improve the efficiency of the frequency spectrum by decreasing the power of the transmitters as much as possible during the connection while maintaining a good connection quality, and secondly, to increase the battery life of the mobile station.
In a mobile system, obstacles over the radio path result in the arrival of several reflected signals at the receiver, causing thus changes in the amplitude. This phenomenon is known as Rayleigh fading, since the amplitude of the sum of several signals with a random distribution of phases is Rayleigh-distributed. Due to the Rayleigh fading, an additional margin is required at the level of the transmission since the power control of the transmitter cannot follow rapid changes in the signal level.
The additional power that is required for compensating for the Rayleigh fading and that is added to the transmissions of both the mobile station and the base station is problematic, since the interference increases disturbance in the reception of the other users, thus decreasing the network capacity. The additional transmit power also decreases the battery life of the mobile station. The aforementioned problems concern both the GSM system and the WLL system based thereon.